Fixed footer accessibility issues and navigation jumping

[bdenham]

Purpose of this pull request

This pull request (PR) improves footer accessibility and preserves sidebar state when navigating between documentation pages.

• Add accessible link labels, new-tab notices, and visible keyboard focus styles.
• Correct the Terms of Use and cookie policy links and update the copyright year.
• Improve footer wrapping and use theme-aware social icon colors.
• Save sidebar expansion and scroll state before navigation and when leaving a page.

Testing

• pnpm build:prod-fast passed: All internal links are valid.
• git diff --check origin/release...HEAD passed.
• Build reported a duplicate /get-started/seo route warning outside this diff.

Affected pages

Site-wide: all documentation pages using the shared footer and sidebar.